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March 1, 2012

Triceratops And Torosaurus: Are They Really Different?

by editor

A debate over whether Triceratops and Torosaurus are two different life stages of the same species has been decided in a new analysis of the prehistoric specimens classifying them into two distinct groups.

The study rejected 2010 research claiming the Triceratops was merely a youngster and Torosaurus was an adult of the same species. The new research, published Feb. 29 in the journal PLoS One, said the analysis of the fossils do not support the same species theory.

Nicholas Longrich and Daniel Field, researchers at Yale University in New Haven, Conn., studied 35 specimens attributed to both species and concluded they represented two distinct animals.

“We looked at a bunch of changes in the skulls as the animals age and used a program to arrange the skulls from youngest to oldest,” Longrich told Neil Bowdler of BBC News. “What we found is there are young Torosaurus individuals and very old Triceratops individuals and that´s inconsistent with Torosaurus being an adult Triceratops.”

Studying the skulls and frills closely, the team found that Torosaurus skulls boasted a longer frill with large holes, while Triceratops had a smaller solid frill. They argue that if these were the same animal, they would also expect to find transitional specimens in which the skull is morphing between the two skull types.

“We reviewed the evidence and there was no evidence for anything between Torosaurus and Triceratops. There are dozens and dozens of skulls and I think if those transitional forms really existed we would have found them,” Longrich noted.

Longrich and Field, however, could not find conclusive evidence for the two species theory when looking at geographical distribution of the fossil evidence. Fossils attributed to both species are found exclusively in North America, and although there are some sited where only one proposed genus has been found, the evidence could have been consistent with the single-species theory.

John Scannella of Montana State University, co-author of the 2010 research, said he was not convinced with the new analysis.

“Nothing in the Longrich and Field paper falsifies the synonymy of Triceratops and Torosaurus,” he told BBC News. “Triceratops and Torosaurus overlap geographically and stratigraphically; Torosaurus are more mature than other Triceratops, as has been demonstrated multiple times by examination of the bone microstructure; and there are numerous intermediate specimens which demonstrate the transition from the solid frill of Triceratops to the expanded, fenestrated (holed) condition observed in Torosaurus.”

Scannella also noted that the Smithsonian in Washington has a transitional Triceratops that confirms the single-species theory.

“It has a small hole in its frill where Torosaurus has a larger hole, but Longrich and Field suggest that it is simply pathological. There are many other transitional specimens, several of which have been collected in recent years by the Museum of the Rockies,” he explained.

Michael Pitman of the Department of Earth Sciences at University College London said that in most tetrapods (four-legged animals) the skeleton changes as it grows; and it is “important that these growth-related changes are taken into account” when studying the differences between Triceratops and Torosaurus.

“The study shows how the skulls of this group of dinosaurs probably grew, and appears to falsify the hypothesis that Torosaurus is an adult Triceratops,” he said. “The methods used in the study have broader value for helping to reconstruct growth series in other dinosaurs, which could potentially help to test similar hypotheses in other dinosaur groups.”

Professor Paul Sereno, a paleontologist at the University of Chicago, who doesn´t typically side with the single-species theories, told BBC News that he feels paleontology sometimes goes too far in eliminating animals “that deserve their status as separate species.”

“Besides Torosaurus not fitting neatly into a geriatric Triceratops hypothesis, the authors also remind us of other features involving the ornamental frill bones and shape of the fenestrae (frill holes) that differ in Torosaurus when compared to similar frill bones and depressions in Triceratops,” he added.

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Image Caption: A year-long study by Yale University paleontologists concludes that two related horned dinosaurs, Torosaurus and Triceratops, are different animals and not adult and juvenile versions of the same. Pictured are Triceratops (top) and Torosaurus (bottom).